Lead-free Sn—Ag—Cu-based solder alloys are mainly used for connection of electric terminals of printed boards or the like. The lead-free Sn—Ag—Cu-based solder alloys are used in various techniques such as flow soldering, reflow soldering and soldering using soldering irons.
Examples of the soldering using soldering irons include soldering by manual operation such as manual soldering. In recent years, the soldering using soldering irons is increasingly automated, and the soldering is automatically performed by iron robots.
The soldering iron is composed of a heating element and an iron tip, and the iron tip is heated by conducting heat of the heating element to the iron tip. In order to efficiently conduct the heat of the heating element to the iron tip, Cu having good thermal conductivity is used for a core material of the iron tip. However, when a solder comes into direct contact with Cu, Cu is leached by Sn in the solder alloy, and a shape of the iron tip is deformed, which makes it difficult to use as the soldering iron. In order to prevent iron-tip leaching caused by Sn, the iron tip is coated with Fe and Fe alloy plating.
As described above, from the viewpoint of prolonging the lifetime of the iron tip, the iron tip is coated with the Fe and Fe alloy plating. However, with an increase in the number of times of soldering due to the automation of the soldering, the leaching has come to occur in the coating on an iron tip surface. The reason why the Fe and Fe alloy plating is leached is that Fe is alloyed with Sn in the solder alloy by mutual diffusion to lead to easy dissolution in Sn in a molten solder. Accordingly, measures on the soldering iron side have limitations, and solder alloys which prevents the occurrence of the Fe leaching have been studied.
Patent Document 1 proposes an alloy in which Co is added to a Sn—Ag—Cu-based solder alloy. According to this document, when Co is added to the Sn—Ag—Cu-based solder alloy, diffusion of Fe into the solder alloy is prevented and an effect of being able to prevent the Fe leaching is obtained.
In addition, as Sn—Ag—Cu—Co-based solder alloys, Patent Documents 2 and 3 propose solder alloys for the purpose of preventing formation of intermetallic compounds or generation of voids, assuming that the solder alloys are used in the reflow soldering.